The present invention relates to an exposure control mechanism for a camera.
More specifically the type of exposure control mechanism of concern is utilized in a single lens reflex camera for controlling an exposure in accordance with the brightness of an object to be photographed. The type of exposure control mechanism of concern includes at least one photosensitive element disposed in a camera body for receiving the scene light reflected at the surface of a leading member of a focal plane shutter and/or the surface of a film loaded in the camera body after the light has passed through an objective lens and a diaphragm. The light impinging on the photosensitive element causes the generation of this element of an output commensurate with the brightness of an object. The above term "and/or" designates the following three cases:
(A) BEFORE THE TRAVEL OF THE LEADING MEMBER, THE PHOTOSENSITIVE ELEMENT RECEIVES SCENE LIGHT REFLECTED FROM THE SURFACE OF THE LEADING MEMBER;
(B) DURING THE TRAVEL OF THE LEADING MEMBER, THE PHOTOSENSITIVE ELEMENT RECEIVES BOTH SCENE LIGHT REFLECTED FROM THE SURFACE OF THE LEADING MEMBER AND SCENE LIGHT REFLECTED FROM THE EXPOSED SURFACE OF THE FILM; AND,
(C) AFTER COMPLETION OF THE TRAVEL OF THE LEADING MEMBER, THE PHOTOSENSITIVE ELEMENT RECEIVES SCENE LIGHT REFLECTED FROM THE SURFACE OF THE FILM.
An exposure control system of this type is shown by U.S. Pat. No. 3,687,026 issued to T. Kobayashi et al.
Exposure control mechanisms of the above-described type have a general advantage that no memory member such as a condensor is required. The memory member is eliminated since there is no need to memorize an output of the photosensitive element after a reflecting mirror of a single lens reflex camera has been moved from its viewing position to its picture taking position. Conventional exposure control mechanisms of the above-described type, however, are constructed for providing either an average light measurement, in which the photosensitive element only receives the light reflected from the entire area of the surface of the leading member and/or the film, or a spot, i.e., partial, light measurement in which the photosensitive element only receives the light reflected from a predetermined portion of the surface of the leading member and/or the film. Several disadvantages are inherent with these systems.
In the conventional exposure control mechanisms of the average light measuring type, the output from the photosensitive element will not be commensurate with the brightness of a main object if the brightness of other surrounding objects changes rapidly during an exposure. This results in an improper exposure on the film of the image of the main object. Such a failure cannot be prevented however even when a possible occurrence of such a rapid change in the brightness can be anticipated for before the exposure operation. In order to prevent such a failure, however, the conventional exposure control devices of this type will require a memory member for memorizing the brightness of the object before occurrence of such a rapid change in the brightness. The requirement of such a memory element, however, means that the previously described general advantage is lost.
On the other hand, in the conventional exposure control mechanisms of the spot measuring type, the output from the photosensitive element will not be commensurate with the brightness of the main object when the image of the main object is formed on an area of the surface of the leading member and/or the film, which is different from the predetermined small area of the spot, because of flaming. In other words, an improper exposure for the main object will be obtained except when the camera is directed so that the image of the main object is formed on the predetermined small area of the surface of the leading member and/or the film. Here again, in order to avoid this drawback, a memory member is also required.